Anti-tumor toxins and cytotoxic drugs have been targeted to tumor cells by directly coupling them to tumor-associated antibodies via a covalent linkage (Gregoriadis, G. et al, Targeting of Drugs With Synthetic Systems, Plenum Press, New York (1986)).
In addition, anti-tumor toxins and cytotoxic drugs have been targeted to tumor cells by indirectly coupling them to tumor-associated antibodies via microspheres (Microspheres and Drug Therapy, Eds. Davis. S. S. et al, Elsevier, New York (1984)). Microspheres are synthetic or natural particles (such as liposomes) having a diameter of up to at least 1.0 .mu.m and include nanospheres which have a diameter of 10 to 100 nm.
However, there are major drawbacks to the above-discussed approaches to the direct and indirect coupling of anti-tumor toxins and cytotoxic drugs to tumor-associated antibodies. More specifically, antibody-toxin conjugates and antibody-cytotoxic drug conjugates arising from direct or indirect coupling are taken up rapidly by macrophages. Kupfer cells and other reticuloendothelial system cells before they can reach the tumor. As a result, less than 1% of the conjugates may actually reach the tumor. In addition, when antibody specificity is not restricted to tumor cells. i.e., if the target antigen is also expressed by a variety of normal cells, the conjugates are not only targeted to tumor cells but are also targeted to normal cells. Thus, since the anti-tumor toxins or cytotoxic drugs which are conventionally coupled to antibodies are highly toxic to normal cells as well as to tumor cells, they cause serious functional damage to normal cell systems, particularly to macrophages and other reticuloendothelial system cells. As a result, while a specific anti-tumor effect is observed in vitro using these conjugates, such a specific anti-tumor effect is less efficiently observed in vivo (Kao, Y. et al. Biochim. Biophys. Acta, 677:453-461 (1981); and Kirby, C. et al. Life Sci., 27:2223-2230 (1980)).
Differentiation-inducers have been observed, in vitro, to change the phenotype of tumor cells so that it is similar to that of non-tumorigenic cells, i.e., to induce "contact inhibitability", low cell saturation density, contact orientation, inability to form colonies in soft agar, etc. (Patt, L. M. et al, Nature, 273:379-381 (1978): Tsao, D. et al. Cancer Res., 42:1052-1058 (1982): Kim, Y. S. et al, Gann Monogr. Cancer Res., 29:93-103 (1983): Sugimura, T. et al, Gann Monogr. Cancer Res., 29:3-15 (1983): Prashad, N. et al, Cancer Res., 47:2417-2424 (1987); and Reuben. L. et al, Int. J. Cancer, 40:224-229 (1987)). However, to date, no study has attempted to similarly alter tumorigenicity or malignancy in vivo. The concentration of differentiation-inducers required to effect the change in the phenotype of tumor cells in vitro is extremely low and not cytotoxic to normal cells. It is more difficult to employ differentiation-inducers in vivo because the necessary systemic concentration of unbound differentiation-inducer can be toxic to specific organs. Thus, to date, there has been no successful application of differentiation-inducers to convert the malignant phenotype to the normal cell phenotype in vivo.
Furthermore, to date, no study has been published or carried out on the targeting of differentiation-inducers via antibodies or any specific ligand directed to tumor cells, either in vitro or in vivo. This is a striking contrast to a number of studies which have been carried out on the targeting of anti-tumor toxins and cytotoxic drugs to tumor cells via antibodies or specific ligands directed to tumor cells. In addition, no study to date has been carried out to ascertain the most efficient way to target differentiation-inducers to tumor cells, either in vitro or in vivo.